Readout modules for vapor and liquid phase SAW sensors fabricated on piezoelectric films are typically configured as single or dual delay line oscillator loops. Mass loading of the sorbent film realized on the SAW device is detected as a frequency shift which is read externally via a frequency counter. However, this approach is not directly applicable in the development of a monolithically integrated autonomous sensor system suitable for wearable sensor tags and other field applications. In this work we have developed a data measurement topology suitable for monolithically integrated SAW sensors on CMOS chips, a technology that is not fully developed and will significantly increase Si-CMOS functionality. This readout technology achieves closed loop conversion of the SAW frequency response to a well-defined output voltage accurately tracking sensor behavior in real time. The topology is appropriate for thin film, low loss interdigitated (IDT) SAW devices used as mass loading sensors, such as those reported in [1] and [2].
The proposed closed loop system is controlled by a finite state machine (FSM) which forces the system output to oscillate within a narrow voltage range that correlates with the SAW pass-band response. The period of oscillation is of the order of the SAW phase delay. We also use timing information from the FSM to convert SAW phase delay to an on-chip 10 bit digital output operating on the principle of time to digital conversion (TDC). The output voltage range varies with changes in SAW center frequency, thus tracking mass sensing events in real time. This architecture precludes mode jumping issues found in designs incorporating the SAW delay line or the resonator in the feedback loop of an amplifier. It was demonstrated that the system can be adapted to alternate SAW center frequencies and group delays by adjusting the VCO control and TDC delay control inputs. Because of frequency to voltage and phase to digital conversion, this topology does not require external frequency counter setups and is uniquely suitable for full monolithic integration of autonomous sensor systems and tags.